New drivers

[The Natural Philosopher]

dukie I got that years ago.

[David WE Roberts]

*What is it, and when are you going to fix it?

Volvo 850 R auto with an alleged 240 bhp when new. There is enough grunt in the engine to get it rolling at tickover and then the gearbox will change up - not unreasonable as road speed increases. Especially on a slight down slope. IIRC my old 3 litre auto Toyota Supra and my old Volvo 760 Turbo auto also exhibited similar characteristics.

None of them do 0-30 in nothing flat without any throttle but there is enough low down power coupled with quite high gearing to get them rolling at quite a lick starting from rest at tickover.

Also, I was not aware that cars were designed to sit at rest in gear without the brakes on. They either stall or start rolling. With a manual you are restricted to 1st gear. With an auto it can change up once it is rolling - as presumably it is not bright enough to know that if it is rolling along with zero throttle it shouldn't change up if you don't want it to. Under normal driving, if you lift off after acceleration the auto box will shift up if you are not in top gear.

Perhaps I should have used the word 'eventually'?

Cheers

Dave R

[The Natural Philosopher]

why would it? the engine fuel is cut till revs drop below 600 or so at which point is restored;

well tickover in first gear is about 3 mph probably so it doesn't happen unless you don't press the clutch when you go below 3mph. I suggest you put your car in first and then take your foot ioff. It slows down until about 3mph and then suddenlly the fuel comes in, and it creeps forwards after a lurch.

[The Natural Philosopher]

It always has some. Its hygroscopic and in fact contains additives to make sure it does absorb it rather than have it corroding the brake system.

Nor will it boil ever on a road car - even

try putting a soldering iron into brake fluid - that's not even red hot.

Yiou will see vapour coming off.

[The Natural Philosopher]

it might well be the sort of auto that starts in 2nd.

I have to say that, held in top most cars WILL just about hold 30mph at tickover.

[The Natural Philosopher]

no. My XKR stayed in first or second..whatever it started in..it would do about 5mph - a good creep speed.

[Terry Fields]

Only the release bearing is involved in a change of gear, and in 50+ years of using engine braking I've never had one go.

I've only had one clutch go, and that was on a secondhand purchase a month previous - and I'd used the fact that it was on the way out to beat the asking price down.

My pads, discs, shoes and drums seem to outlast everyone else's, for some reason...

[Tim Watts]

Interesting

Remember it well... My second paid-for old banger (Maestro) had gearbox oil that went between tar and water depending on box temperature. Solved that with some nice Millers oil, but 2nd synchro was pretty shot and an elemnet of double declutching was advisable. Good job I'd practiced on my mate's Serial IIA landy which never had synchro on 1+2

[Reentrant]

I know that's always given as a reason for brake fade, but in theory that should *increase* the pressure in the slave cylinder and improve braking. The gas is compressible, but won't it occupy more volume than the original liquid that boiled?

What is the typical pressure in the hydraulic system when you're braking hard, and what is the boiling point of water at that pressure? It's going to be way over 100C.

[Roger Chapman]

The way I see it it is still how hard you press the pedal that determines the pressure at the slave piston so would the steam pressure initially force your foot upwards and eventually move the master piston to a point where the way is open to the reservoir. Exit much fluid from the connection to the brakes and next time you press the pedal down it goes to the floor.

[Jules Richardson]

Well, yes - "perfectly adequate" does not equate to "good", though :-) Parts will wear, but at different rates in different vehicles - and in certain Triumphs it seems it was "very soon".

After some googling, it seems that the problem is a bit of a design one - as well as pulling horizontally, due to the path that they take the handbrake cables have a certain amount of vertical pull, too; in a new setup this isn't a problem, but after a short time it wears the pivot points (which sound like they weren't particularly up to the job) for the operating levers in the back wheel assemblies. The net result is that a lot of the handbrake's movement goes into picking up the "vertical" slack in the pivots rather than actually moving the brake shoes.

The fix (as in "longer term until it needs maintainance again") seems to be to drill out the original pivots and pin them with something a bit more robust.

cheers

Jules

[Roger Chapman]

The way I see it it is still how hard you press the pedal that determines the pressure at the slave piston so would the steam pressure initially force your foot upwards and eventually move the master piston to a point where the way is open to the reservoir. Exit much fluid from the connection to the brakes and next time you press the pedal down it goes to the floor.

[Dave Plowman (News)]

Regardless of 'how much grunt' an engine may have at full throttle, it only produces enough power at idle to overcome the losses of its own friction and the things it drives at that time. If it produced more than this, its speed would increase and increase... That's why most these days have a closed loop idle mechanism which keeps the idle constant regardless of the load of the alternator, air-con, etc. But no auto I know of will change up at idle revs. Otherwise it would change up several times before moving off...

Well, down a slope a manual could reach well over 30 mph without ever being put in gear...

So around town you have to keep the brakes on permanently to avoid breaking the speed limit? Sounds rather inefficient to me.

Quite a lick now? If you made that a fast walking pace on the level I'd agree with you. But not 30 mph.

Sounds like your auto is faulty in more than one way.

It knows both the engine RPM and the road speed. Even very early ones did. And it will only change up when both those pre-sets have been exceeded.

Depends how fast you're going. My BMW won't select top gear until about 60 mph regardless of where the throttle is. And there are similar minimums for all gears when in auto.

I'd suggest you find a flat bit of road with no traffic and actually try it.

[The Natural Philosopher]

The point is that the hydraulic fluid is not under any pressure when the brakes are off, that means the fluid can and does get pushed back into the reservoir, leaving gas bubbles in the slave cylinders. Even if the gas is then fully compressed you may run out of brake pedal travel before you have a decent pressure which is in fact extremely high.

I.e. it's not the first hard stop that boils the fluid/water that causes the issue, the NEXT time you try the pedal just goes to the floor.

As if you hadn't bled the brakes

But mainly its pad glazing - I did an emergency stop from about 100mph+ on my Opel Manta years ago.. by the time it got below 30mph the brakes were unable to get anywhere near locking the wheels - pedal pressure remained constant throughout.

[Tim Watts]

Until you release the pedal, the gas expands, pushes the fluid back into the reservoir and now you have a gas bubble in your hydraulics!The volume of fluid moved to actuate the brakes in a car is not very much, so having more than the smallest of gas bubbles tends to bugger things.

[Andy Dingley]

Not any more. It used to be listed as a scheduled service item (although I don't recall this ever happening on any service I ever paid for). These days though, the practice is to test the fluid's boiling point and only replace it if it's too low.

[The Natural Philosopher]

Depends on the car.

Fairly sure it was part of the jaguar 2 or 5 year service.

[Huge]

[Jules Richardson]

Northern Minnesota, US - home to ridiculous amounts of snow and deer (I think there's about one deer to every five people, and around 6% of all reported crashes are due to deer impacts)

cheers

J.

[charles]

but the fluid is hydroscopic - it takes up water from the atmosphere